It has long been known that a liquid manometer can be used to measure the pressure of gases that obey Boyles Law by compressing the gas a known amount prior to making the measurement. The McLeod gauge, once widely used for measuring low pressures, utilizes this principle, as discussed in, J. H. Leck, Pressure Measurement in Vacuum Systems, Second Edition (Chapman and Hall, London 1964) pp. 3-7. A mercury piston is used to compress gas trapped in a known volume by a measurable amount thereby permitting the unknown pressure to be calculated. In one mode of operation, both the compression ratio and pressure differential created by compression of the gas are variables. In the second mode, the compression ratio is fixed but the pressure differential is a variable. At best, a McLeod gauge is a fragile, large, cumbersome gauge which requires considerable operator skill to manually make a measurement of pressure. A single pressure measurement is carried out over an extended period of time in that the mercury must be transferred slowly to avoid disastrous results. Also, mercury vapor is a health hazard if inhaled, ingested, or absorbed through the skin, and mercury vapor is anathema in modern day vacuum systems. However, mercury or other liquid which does not wet the inner surfaces is required to prevent leakage of the compressed gas in a McLeod gauge. McLeod gauges have been largely replaced by modern capacitance manometers.
As is known, diaphragm vacuum gauges measure absolute pressure independent of gas species. Such gauges which measure force per unit area are typically used for measuring low pressures of gas mixtures where measurement of ionization currents in an ionization gauge or heat lost from a hot wire are ineffective: where high accuracy is required; or of corrosive or hostile gases. At low pressure, the force per unit area exerted by the gas molecules on a surface is extremely small so very thin diaphragms must be used to achieve the required sensitivity. Producers of such gauges are now providing 1 and 0.1 Torr full scale sensors with 4 to 5 decades of dynamic range in an attempt to satisfy user requirements. However, the force per unit area exerted on a surface by a gas at, for example, 1.times.10.sup.-4 Torr is only about 2.times.10.sup.-6 psi. It is apparent that a gauge capable of measuring such a small force per unit area will be extremely sensitive to mechanical and thermally induced stresses. Even a slight overpressure will cause a significant zero shift in such instruments incorporating these sensitive diaphragms.
U.S. Pat. No. 4,413,526 to Delajoud issued Nov. 5, 1983, discloses a device for measurement of fluid pressures including a vertical cylinder, a piston adapted to slide in the cylinder with viscous friction with the pressure to be measured being applied to the upper face of the piston which rotates the piston inside the cylinder. An electromagnetic precision weighing machine including a shaft and piston arrangement to which the pressure to be measured is applied is used to measure the force on the piston. The gas pressure which is to be measured acts on the piston, the force of which corresponds to the product of the affected area of the piston and the pressure difference across the piston. Devices such as this are not useful in most vacuum measurement applications because of gas leakage.
In an effort to overcome some of the above-noted shortcomings, a capacitance manometer was developed and disclosed in U.S. Pat. No. 4,823,603 to Ferran et al. issued Apr. 25, 1989. Therein, a capacitance manometer having stress relief for a fixed electrode including a thin electrically conductive diaphragm fixedly mounted to a housing comprising an electrode of a variable capacitor is disclosed. While the manometer set forth therein solves the problem associated with capacitance changes caused by temperature and a stray capacitance caused by leakage currents through a dielectric ceramic material, the disclosed manometer is still very sensitive to mechanical disturbances and requires a reference pressure for absolute pressure measurement.
In yet another attempt to overcome the above noted shortcomings, U.S. Pat. No. 5,022,207 to Rud Jr. issued Jun. 11, 1991, discloses a transmitter having a pressure sensor for sensing pressure and an overpressure protection means for limiting the pressure applied to the pressure sensor when the applied pressures exceeds a preselected limit.
Clearly, there is a need for a gauge which can measure low pressures without being sensitive to mechanical or thermal stresses or to overpressure. Further, it is desirable to measure even lower pressures than can be measured today with typical sensitive diaphragm gauges. It would also be desirable to be able to utilize a single diaphragm thickness over a wide pressure range rather than resort to installing a separate gauge for each pressure range, and to utilize a differential diaphragm manometer rather than an absolute manometer to measure absolute pressure. These needs and advantages can be achieved in accordance with the present invention and will become apparent from the following description.